One means of providing a bright metal finish on a molded plastic item is to sputter a thin layer of chromium or chromium-containing alloy onto its surface. Herein, the term chromium or chrome layer refers to a thin layer of chromium or chromium-containing metal applied to a substrate by vacuum sputtering methods. A sputtered layer only a few microns thick can provide an appearance equivalent to that of a like article with a substantially thicker electroplated chromium coating. Generally, a clear, glossy topcoat is applied over a sputtered chromium layer to prevent wear by abrasion and exposure to corrosive environmental conditions.
Sputtered metal coatings may not adhere strongly enough to a plastic substrate to endure the type of adverse environmental conditions encountered by exterior and interior automotive trim parts. Because the coefficients of thermal expansion of plastic and sputtered metal layers are very different, thermal cycling may lead to delamination of a coating. Flexing a pliable or elastomeric substrate may cause a sputtered layer to crack and peel. The initial adhesion of a sputtered chromium-containing coating may also be adversely affected by such conditions as inadequate cleaning of the substrate, the presence of mold release agents, the presence of solvent in the plastic substrate or base coat, incompatability of the plastic and metal, or other conditions brought about by the composition of the constituents or processing conditions.
Thus, means have been sought to consistently provide for good adhesion between sputtered metal and plastic substrates. We have developed a method of improving adhesion which mitigates such adhesion-inhibiting conditions without addition of costly equipment or time consuming steps.